OTOH, if you don't want to buy HoH: True Lineages, I think your definately on the right track... though you might want to consider that detecting a sigil represents a fairly detailed peice of information. Check the second paragraph in the guidelines for InVi for information concerning detecting specific details about a spell.
By laying your hands on the belly of a pregnant woman, the spell will tell if the baby is tied to the magical realm or not.
The spell is not strong enough to detect if the child will have the full hermetic gift or not nor will it detect if the child is tainted by another realm. It will simply reveal the link to the magical realm. Strong links will be reveiled instantly while more subtile links will only tinkle the ends of the touching fingers.
Augustus of Criamon used this spell during his travels & found a number of suitable aprentice for the order doing so. He soon got the reputation of beeing a softy babysitter. Life was a mystery for Augustus & he was sure that the answer lied in the mouth of the young ones.
(Base 5, +0 Per, +1 Conc, +1 Touch, rq. Free, +2 Extra sensitivity(Gentle gift & Un-revealed gift)